The present invention relates to semiconductor photodetector devices, and particularly to a semiconductor photodetector device in which a photodetector and a logic element are formed on the same substrate.
Opto-electronic integrated circuit (OEIC) devices in each of which a photodetector such as a photodiode converting a light signal into an electric signal and a logic element such as a transistor and a capacitor constituting a peripheral circuit are formed on a substrate are known to date as semiconductor photodetector devices, and have been used as optical pickup devices for compact discs (CDs) or digital versatile discs (DVDs), for example.
Hereinafter, an OEIC device including a transistor and a photodiode will be described as a first conventional example.
FIG. 6 shows a cross-sectional structure of an OEIC device according to the first conventional example. As shown in FIG. 6, in the OEIC device of the first conventional example, an n-type semiconductor layer 202 with a low impurity concentration and a thickness of about 4.4 μm is epitaxially grown on a substrate 201 of silicon containing a p-type impurity at a low concentration. In the substrate 201 and the n-type semiconductor layer 202, an isolation region 203 is formed as a p-type buried region with a high impurity concentration, thereby defining a region where an npn-bipolar transistor 20 is formed and a region where a photodiode 40 is formed. On the n-type semiconductor layer 202, an insulating layer 204 having contact holes for making contacts with electrodes of the bipolar transistor 20 and the photodiode 40 is formed.
The bipolar transistor 20 includes: a collector-lower-part extraction region 221 that is a high-concentration n-type buried region formed between the substrate 201 and the n-type semiconductor layer 202; a collector region 222 formed by diffusing an n-type impurity in the n-type semiconductor layer 202; a base region 223 formed by selectively diffusing a p-type impurity in the collector region 222; an emitter region 224 and a base contact region 225 formed by selectively diffusing an n-type impurity with a high concentration and a p-type impurity with a high concentration, respectively, in the base region 223; and a collector-upper-part extraction region 226 and a collector contact region 227 formed by diffusing a p-type impurity in respective parts of the n-type semiconductor layer 202 at a side of the base region 223. In the bipolar transistor 20, the emitter region 224, the base contact region 225 and the collector contact region 227 are connected to an emitter electrode 228, a base electrode 229 and a collector electrode 230, respectively, via contact holes formed in the insulating layer 204.
The photodiode 40 includes: a cathode region 241 that is a high-concentration n-type buried region formed between the substrate 201 of p-type silicon and the n-type semiconductor layer 202; an anode region 242 formed by diffusing a p-type impurity with a high concentration in the n-type semiconductor layer 202; a cathode extraction region 243 and a cathode contact region 244 formed by diffusing an n-type impurity with a high concentration in respective parts of the n-type semiconductor layer 202 at a side of the anode region 242. In the photodiode 40, the anode region 242 and the cathode contact region 244 are connected to an anode electrode 245 and a cathode electrode 246, respectively, via contact holes formed in the insulating layer 204.
In the OEIC device of the first conventional example, when a reverse bias voltage is applied across the anode electrode 245 and the cathode electrode 246, a depletion layer is formed at the pn junction between the high-concentration anode region 242 and the low-concentration n-type semiconductor layer 202. This depletion layer is formed predominantly in the low-concentration n-type semiconductor layer 202. A depletion layer is also formed at the pn junction between the high-concentration cathode region 241 and the low-concentration substrate 201 in the same manner. In these two depletion layers, carriers are generated from incident light entering from the upper face of the photodiode 40, thereby generating photoelectric current. The amount of this photoelectric current increases in proportion to the amount of incident light absorbed in the n-type semiconductor layer 202. The amount of absorbed incident light depends on the distance in which the incident light travels from the surface of the semiconductor substrate toward the inside. The amount of absorbed incident light is represented as 1−exp (−αt) where t is the depth from the surface and α is the absorption coefficient of the incident light. The depletion layer extending from the high-concentration cathode region 241 to the low-concentration substrate 201 is useful in capturing carriers generated from incident light deeply entering the inside across the cathode region 241 and effectively increases light sensitivity to laser light with a relatively long wavelength (e.g., infrared laser light at about 780 nm.) However, carriers that have reached a deep part of the substrate 201 are added as a part of photoelectric current, so that these carriers are diffused and move in the substrate 201 for a longer time. This causes deterioration of high-frequency characteristics of the photodetector.
As a second conventional example, as shown in FIG. 7, an OEIC device in which a first p-type semiconductor layer 251 containing an impurity at a high concentration and a second p-type semiconductor layer 252 containing an impurity at a low concentration are formed between a substrate 201 and an n-type semiconductor layer 202 has been developed.
In the OEIC device of the second conventional example, carriers generated from incident light reaching a portion deeper than the first p-type semiconductor layer 251 are absorbed in the first p-type semiconductor layer 251 containing the high-concentration impurity and do not reach a cathode region 241, so that the frequency response characteristic of a photodetector is not lost. On the other hand, in a depletion layer formed in the second p-type semiconductor layer 252 between the cathode region 241 and the first p-type semiconductor layer 251, carriers generated from strong incident light which is hardly attenuated are captured as photoelectric current, so that the device of the second conventional example hardly losses light sensitivity, thus enhancing frequency characteristics as compared to the device of the first conventional example.
In recent years, in the case of applying such an OEIC device to an optical pickup device, higher light sensitivity of the photodiode 40 and enhancement of operating speeds and frequency characteristics of the bipolar transistor 20 and the photodiode 40 have been demanded. As a light source for an optical pickup device, laser light with a wavelength of about 780 nm and a laser light with a wavelength of about 650 nm are used for CDs and DVDs, respectively, for example. In more recent days, to increase recording density, light sensitivity to blue light with a wavelength of about 410 nm is also demanded, as described above.
However, in each of the OEIC devices of the first and second conventional examples, the n-type semiconductor layer 202 serves as an active region and the collector region 222 located in an upper part of the n-type semiconductor layer 202 serves as an active region of the bipolar transistor 20. Accordingly, if the thickness of the n-type semiconductor layer 202, for example, is increased to 5 μm or more, the light sensitivity of the photodiode 40 increases but the thickness of the collector region 222 increases as the thickness of the n-type semiconductor layer 202 increases. As a result, there arises a problem in which high-frequency characteristics of the bipolar transistor 20 are impaired. In view of this, if priority is given on improvement of high-frequency characteristics of the bipolar transistor 20 and the thickness of the n-type semiconductor layer 202 is reduced to 1 μm or less, parasitic capacitance due to isolation of the bipolar transistor 20 decreases, so that the high-frequency characteristics of the bipolar transistor 20 are improved but the light sensitivity of the photodiode 40 decreases.
In the photodiode 40 of each of the first and second conventional examples, carriers generated from incident light are captured in the depletion layer at the pn junction between the p-type anode region 242 and the n-type semiconductor layer 202 and the depletion layer at the pn junction between the n-type cathode region 241 and the p-type substrate 201, so that junction capacitances of these pn junctions increase, and a transfer characteristic (gain) at high frequencies is not ensured. In the second conventional example, the high-frequency characteristics are improved by the presence of the first p-type semiconductor layer 251 as compared to the first conventional example. However, the influence of the junction capacitances of the pn junctions prevents sufficient improvement of the high-frequency characteristics even in the second conventional example. In addition, the photodiode 40 and the bipolar transistor 20 of each of the first and second conventional examples are isolated such that the n-type semiconductor layer 202 and the collector region 222 serving as their active regions are surrounded by the isolation region 203 including the pn junctions. Accordingly, the pn junction capacitances due to the isolation are added to these elements, so that a cause of deterioration of high-frequency characteristics arises.
The percentage of absorption of blue light is about 60% in the portion from the surface to a depth of about 0.2 μm and decreases in a deeper portion as the amount of incident light which has reached the portion (i.e., the amount of absorbed light) increases. In the first and second conventional examples, the high-concentration p-type base region 223 and the high-concentration p-type anode region 242 are generally formed by the same diffusion process to share the diffusion process, so that the diffusion depth for the base region 223 and the anode region 242 is about 0.6 μm. The depletion layer formed at the pn junction between the high-concentration p-type anode region 242 and the low-concentration n-type semiconductor layer 202 is also present in the anode region 242 and extends toward the surface of the anode region 242. However, this distance is about 0.2 μm at most. Accordingly, in the structures of photodetectors of the first and second conventional examples, light is received at a position at which the amount of absorbed blue light is small, so that sufficient light sensitivity to blue light is unlikely to be obtained.
In view of this, the thickness of the anode region 242 needs to be smaller than 0.2 μm so as to cause carriers to be generated from incident light in a region where the light-conversion efficiency is high. However, in the OEIC devices of the first and second conventional examples, if the thickness of the anode region 242 is reduced, a sufficient depletion layer is not formed in the n-type semiconductor layer 202, so that it is difficult to enhance light sensitivity to light with a short wavelength.
In addition, an OEIC device used in a recent optical pickup device needs to not only maintain previously-achieved light sensitivity and high-speed response to infrared light and red light but also satisfy light sensitivity and high-speed response to blue light at the same time.
Hereinafter, an OEIC device in which a photodiode and a bipolar transistor are formed in a monolithic manner will be described as a third conventional example (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-219534.)
FIG. 8 shows a cross-sectional structure of an OEIC device in which an npn-transistor serving as a bipolar transistor and a photodiode of an anode common type are formed on the same substrate.
As shown in FIG. 8, a p+-type semiconductor region 302 of a high-concentration semiconductor layer is formed on a semiconductor substrate 301 having a specific resistance of 150 Ωcm and made of p-type silicon (Si) with a low impurity concentration. On the p+-type semiconductor region 302, a p−-type semiconductor region 303 having an impurity concentration lower than that of the p+-type semiconductor region 302 is formed. On the p−-type semiconductor region 303, an n-type semiconductor region 304 having an impurity concentration higher than that of the p−-type semiconductor region 303 is formed.
The peak of the impurity concentration in the p+-type semiconductor region 302 is set at a depth of about 10 μm from the upper face of the n-type semiconductor region 304. The thickness of the n-type semiconductor region 304 is set equal to or smaller than that of an isolation insulating layer 305. For example, if the thickness of the isolation insulating layer 305 is 1 μm, the thickness of the n-type semiconductor region 304 is set at 1 μm or less.
A photodetector part 100 and a transistor part 200 are defined in the p−-type semiconductor region 303 and the n-type semiconductor region 304. An n+-type semiconductor region 306 having an impurity concentration higher than that of the n-type semiconductor region 304 is formed in the uppermost part of the n-type semiconductor region 304 in the photodetector part 100. In this case, the thickness of the n+-type semiconductor region 306 is 0.15 μm or less.
A cathode of the photodetector part 100 is constituted by: a cathode contact region 307 formed in the periphery of the n+-type semiconductor region 306; an n-type polycrystalline semiconductor layer 308; and a cathode electrode 309. An anode of the photodetector part 100 is constituted by: a p+-type buried region 310 formed in the periphery of the photodetector part 100; an anode contact region 311; a p-type polycrystalline semiconductor layer 312; and an anode electrode 313.
On the other hand, the transistor part 200 made of an npn-bipolar transistor is formed in the n-type semiconductor region 304 and isolated by the isolation insulating layer 305 and the p+-type buried region 310. A collector of the transistor part 200 is constituted by: a buried collector region 314; a collector contact region 315; the n-type polycrystalline semiconductor layer 308; and a collector electrode 316. A base is constituted by: an active base region 317; a contact base region 318; the p-type polycrystalline semiconductor layer 312; and a base electrode 320. An emitter is constituted by: an emitter region 319; the n-type polycrystalline semiconductor layer 308; and an emitter electrode 321.
Operation of the conventional semiconductor device configured in the manner described above will be described with reference to FIGS. 8, 9A and 9B.
First, the surface of the n+-type semiconductor region 306 is irradiated with light incident on the photodetector part 100. As shown in FIGS. 9A and 9B, carriers generated in the n+-type semiconductor region 306 and the n-type semiconductor region 304 are accelerated by a potential gradient a formed by the concentration difference between the n-type semiconductor region 304 and the n+-type semiconductor region 306 to move from the n+-type semiconductor region 306 to a flat region c of the n-type semiconductor region 304 without disappearing due to recombination. These carriers reach the p−-type semiconductor region 303. In the p−-type semiconductor region 303, a reverse bias voltage is previously applied to the cathode electrode 309 of the photodetector part 100, so that a depletion layer is formed in a region extending from the p−-type semiconductor region 303 surrounded by the p+-type buried region 310 in the periphery of the photodetector part 100 to the p+-type semiconductor region 302. Accordingly, the carriers that have reached the p−-type semiconductor region 303 move in the depletion layer at high speed as drift current, thus enabling high-speed response of the photodetector part 100.
Incident light which has reached the semiconductor substrate 301 is converted into carriers in the semiconductor substrate 301 and these carriers move freely by diffusion. The speed of this movement is slow because this movement is caused by diffusion, and the carriers partly disappear by recombination. Carriers which do not disappear by recombination reach a portion near the p+-type semiconductor region 302. However, these carriers do not reach the p+-type semiconductor region 302 and the p−-type semiconductor region 303 contributing generation of current because of the presence of a potential barrier formed by the impurity-concentration difference between the p+-type semiconductor region 302 and the semiconductor substrate 301, and recombine together to disappear. Accordingly, it is possible to have carriers moving by diffusion disappear, thus enabling a higher response speed.
However, the third conventional example has a problem in which the n-type semiconductor region 304 has a small thickness of 1 μm or less, so that a vertical pnp-transistor (VPNP-Tr) exhibiting high-speed response is not formed. Accordingly, a transistor of an OEIC device is limited to an npn-transistor. This is because, as shown in FIG. 8, if the type of the buried collector region 314, which is “n” in the case of an npn-bipolar transistor constituting the transistor part 200, is to be changed to “p”, it is necessary to provide an n-type buried layer for separation between the buried collector region 314 and the p−-type semiconductor region 303.
FIG. 10 shows an OEIC device in which a VPNP-Tr is formed in the device of the third conventional example, as a reference example for solving the problem arising in the third conventional example. As shown in the cross-sectional structure in FIG. 10, in this reference example, the VPNP-Tr is formed as a second transistor part 220. In this example, an n-type buried layer 330 is formed in part of the p−-type semiconductor region 303 in the second transistor part 220 and a p-type buried collector region 331 is formed in an n-type semiconductor region 304 formed on the p−-type semiconductor region 303. The thickness of the n-type semiconductor region 304 is set at about 2 μm so as to form the p-type buried collector region 331 as intended.
A collector of the second transistor part 220 is constituted by: the p-type buried collector region 331; a collector contact region 332; an n-type polycrystalline semiconductor layer 308; and a collector electrode 333. A base is constituted by: an active base region 334; a contact base region 335; a p-type polycrystalline semiconductor layer 312; and a base electrode 336. An emitter is constituted by: an emitter region 337; the n-type polycrystalline semiconductor layer 308; and an emitter electrode 338. In this manner, the structure shown in FIG. 10 makes it possible to form a VPNP-Tr as the second transistor part 220.
As described above, an optical pickup device has three types including a type for high-density DVDs using blue light as well as a type for CDs using infrared light and a type for DVDs using red light, and needs to have sufficient light sensitivity and high-speed response to blue light. For example, the depth from the incident light face at which the amount of light absorbed in silicon semiconductor is about 90% is about 24 μm in the case of infrared light with a wavelength of 780 nm, is about 7.7 μm in the case of red light with a wavelength of 650 nm and is about 0.6 μm in the case of blue light with a wavelength of 407 nm. With respect to characteristics of a photodetector device, a structure in which carriers contributing generation of photoelectric current are effectively extracted as a light absorption amount depending on the wavelength of light enables enhancement of light sensitivity and response speed.
On the other hand, in a logic element part, a VPNP-Tr exhibiting high-speed response needs to be provided because of an increased operating frequency of a semiconductor integrated circuit.
However, in the OEIC device of the third conventional example, in the case of receiving blue light, the thickness of the n-type semiconductor region 304 is 1 μm or less, so that it is difficult to define a region where the p-type buried collector region 331 in the VPNP-Tr in the n-type semiconductor region 304 as intended.
In the reference example shown in FIG. 10, if the thickness of the n-type semiconductor region 304 is 1 μm or more, a flat region d where the potential gradient to electrons is flat becomes dominant as shown in FIG. 11B, so that the traveling distance in which carriers move in the flat region d is long. Accordingly, the time required for carriers to reach a depletion layer formed between the p−-type semiconductor region 303 and the n-type semiconductor region 304 is long, so that the response speed decreases. As the traveling distance of carriers increases, the amount of recombination increases, so that light sensitivity decreases.
Accordingly, as understood from the structures of the third conventional example and the reference example, the mounting of a VPNP-Tr and operation characteristics of a photodetector have a trade-off relationship.
As another problem, as shown in FIG. 9B, frequency characteristics of the photodetector part 100 deteriorate because the resistance value due to series resistance resulting from a low concentration layer e formed between the p+-type buried region 310 and the p+-type semiconductor region 302 is high in the p−-type semiconductor region 303 of the semiconductor photodetector device of the third conventional example. In this conventional example, to reduce this series resistance, a pattern is formed in such a manner that the p+-type buried region 310 is relatively wide. However, this enlarges a peripheral region of the photodetector part 100, and a problem in which the chip area cannot be reduced arises.